I Corps was the northern most area of operations, nearest the demilitarised zone between North and South Vietnam. I Corps became the main operational area for the USMC due to this area being envisaged for strategic operations by the Marines in the Operational Plans for the Defence of Mainland South East Asia, made prior to 1964. This is in part, due to the Marines ability to deploy quickly and operate independently. Once deployed, however, the subsequent tempo of operations in I Corps consumed most of the USMC operational capability, limiting their presence elsewhere.

From 1962 US Marine advisors and a Marine Helicopter squadron had been assisting the Vietnamese Marine Corps in limited counter insurgency and riverine operations in the Mekong Delta. The 1964 Bien Hoa airbase attack drove discussions to deploy US main force elements to Vietnam to protect the US air base in Da Nang from any similar enemy action. This was intended to be an initial one off deployment to protect the base, with the added benefit of freeing up ARVN units to conduct operations against enemy forces. There was some discussion at the time of deploying an Army Airborne force for the task of protecting the base, to reduce the diplomatic impact of a Marine Brigade Landing Team rolling off the beach into Da Nang. This was overruled by the Joint Chiefs due to the self-sustaining nature of Marines forces and the overall strategic concern of moving airborne capacity off of Okinawa.

As the situation evolved in Vietnam, the Marines became fully committed to operations in the I Corps area. The short term mission to protect the Da Nang airfield had expanded as the air traffic to this base increased and further airbases were required. The Marine protection role expanded to cover these new requirements.

By the time US operations expanded to include ground forces in the Mekong Delta, the Marines were providing a critical strategic buffer to the area below the DMZ and no additional Marine capacity could be spared for the developing riverine forces. It was noted at the time that the USMC would be more suitable than the Army in this new riverine force, due to their history of interoperability with the navy, but only the Army could spare the resources. Moving the Marines out of I corps into III Corps would also pose serious logistic and security concerns. Moving additional Marine units from Okinawa would also adversely affect strategic balance in the region.

The expansion of US operations in the Mekong Delta necessitated a new approach to warfare, based in part on the riverine operations conducted by the Vietnamese Marines. The task fell to the United States Navy and the US Army, who developed specialised riverine equipment and operational approaches to meet the special requirements of the delta waterways.

The creation of the Special landing team by the US Marines led to expanded amphibious capability throughout I Corps. These special units would also conduct operations across Vietnamese coastal regions as a part of the DECKHOUSE series of operations. DECKHOUSE V was an amphibious operation conducted by the Marines in the Mekong Delta in 1967. This operation demonstrated the lack of suitability of the amphibious doctrine to delta operations and saw only limited success. Issues arose with the shallow water being unsuitable for fire support vessels, hence these vessels were situated too far offshore from the landing area. The tidal nature of the area meant that some deeper draft vessels would have to beach upon sandbars until the tide rose, limiting their mobility during the operation. This operation highlighted the effectiveness of the Navy and Army cooperative riverine operations over more traditional amphibious operations.

The main Corps Areas were a development of the Vietnamese military. Initially there were only three Corps areas, with the Mekong delta and the southern tip of Vietnam making up III Corps. This would change as the war progressed with the southern tip region becoming IV Corps, and III Corps becoming exclusively the Mekong Delta region. This highlights the importance and complexity of the region.

The USMC became well adapted to the mix of operations required over the five provinces in I Corps, largely due to their approach to warfare. Their vertical envelopment and amphibious landing capability were more useful in the I Corps region than in the Delta, as suggested by the lessons learned from the Mekong Delta helicopter operations in the advisory phase of the war, and the amphibious DECKHOUSE operations in 1967. As the focus of III Corps became dedicated to the Delta region, the Army and Navy riverine operations became more specialised to their assigned task, limiting any potential need for Marine integration.

The initial short term commitment by the USMC in I Corps, had soon developed into large scale operations across the region. As the US commitment in South Vietnam grew, the Marines did not have extra capability to commit to the Mekong Delta, despite their suitability for operations with the Navy.

Sources:

U.S. Marines in Vietnam : fighting the North Vietnamese 1967, by Gary L Telfer; Lane Rogers; V Keith Fleming.

The Marines in Vietnam, 1954-1973: An Anthology and Annotated Bibliography, by the

United States Marine Corps. History and Museums Division.

Marine Advisors With The Vietnamese Marine Corps, by Charles D Melson.

Stygian myth: U.S. riverine operations against the guerrilla, by Mark Freitas and Braddock W. Treadway

Marines And Helicopters 1962-1973, by William R Fails.

CINCPAC Command History 1962, Headquarters of the Command in Chief Pacific, 1963.

The Brown Water Navy in the Mekong Delta: COIN in the Littorals and Inland Waters, by Richard E Sessons.

Special study of mobility in the Mekong Delta area of South Vietnam, by George Brinton, Kenneth Clare and Irving Dow.

Game Warden by Victory Daniels and Judith C Erdheim.